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Prevention of Unauthorized Use of a Credit Card

Publishing Venue

IBM

Related People

Gaston, CA: AUTHOR

Abstract

One of the ways of determining proper custody of a credit card is to have an N-digit secret number or word associated with the card. The secret number known only to the card holder must be presented with the card and is decoded to match a validation code on the card. With prior secret number systems, a thief with a stolen card and a validation machine can readily determine the secret number because there is an easily determined reverse transformation from the validation code to the secret number. The following system overcomes this deficiency. The new decoding system is a one-way transformation of the N-digit secret number to a validation code, the transformation has no pattern and there is no reverse transformation from the validation code back to the secret number.

Country

United States

Language

English (United States)

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Prevention of Unauthorized Use of a Credit Card

One of the ways of determining proper custody of a credit card is to have an
N-digit secret number or word associated with the card. The secret number
known only to the card holder must be presented with the card and is decoded to
match a validation code on the card. With prior secret number systems, a thief
with a stolen card and a validation machine can readily determine the secret
number because there is an easily determined reverse transformation from the
validation code to the secret number. The following system overcomes this
deficiency. The new decoding system is a one-way transformation of the N-digit
secret number to a validation code, the transformation has no pattern and there
is no reverse transformation from the validation code back to the secret number.
The lack of a reverse transformation forces a thief to determine the secret
number by trial and error. The lack of a pattern in a transformation means there
is no indication of whether the trial secret number is close to the actual secret
number. This prevents a person from determining the secret number quickly by
following a trial and error path of increasing similarity between the validation code
on the card and the validation code generated from the trial secret number.
There are several ways of implementing this system.

An N-digit validation code is produced by the following transformation
scheme, although a validation code having fewer or more digits can be produced
in the same way. The figure is a table which produces the transformation. The
table has N columns and 10 rows, one row for each of the numerals zero through
nine. To accommodate those people who prefer to memorize a word, rather than
a number, several letters of the alphabet are associated with each numeral. N,
the number of columns, is determined by the degree of security desired and is
equal to the number of digits in the secret number. To decode the secret
number, the number in the row corresponding to the first digit of the secret
number is extracted from the second column of the table; etc. The numbers
extracted from the table are added, discarding carries. The number manipulation
may be done by hand or automatically by machine. The decoding of SAFETY
and SAFELY are show...